I started my wallet today and it started fine - but like 10 minutes later while it was running I got an error and had to close the task. Now when I start it it stucks at "activating best blockchain" and doesn't go further.

Is there any possibility to get it running again without reindexing or reloading the whole blockchain?

Ahh I just had to wait. It took 45 minutes - but now it started without problems and synced.

Thank you anyways

You should update to 0.12 regardless. On windows systems anti virus software can often find signatures of viruses in the blockchain data. The software will then lock the files, which will cause the bitcoin client to crash. There is a fix that comes with 0.12 as it stores the data obfuscated so the virus signatures (there are actually some in the blockchain, but they cant harm you) do not get detected.